Riding lawn mowers and similar vehicles are generally driven by hydrostatic transmissions. Specifically, such vehicles use dual path hydraulic transmissions. Each transmission consists of an over-center variable displacement pump and a fixed displacement motor. The input shaft of the pump is coupled with an internal combustion engine and the output shaft of the motor is coupled with a vehicle wheel. Changing the flow rate and direction of the pump flow will cause the change of rotation speed and the direction of rotation of the wheel.
Any shock load, e.g. in the form of an impediment to the rotation of the wheels, can impart a shock load to the entire hydraulic transmission. This shock load, typically in the form of excessive pressure, can deleteriously affect the components to the system. Many prior art designs of hydraulic transmissions do not integrate parts, e.g. relief valves, into the system in order to protect these components.
Some prior art system designs that have incorporated relief valves into the hydraulic transmission have positioned the valve within the pump. This type of design will protect the pump from shock loads but will not protect the other components, e.g. the motor, that first experience the shock. Specifically, if the shaft of the motor experiences any sudden resistance, or load, a surge of pressure initially will travel through the motor, possibly causing damage. The pressure spike may also damage all other componentry between the motor and pump prior to being dissipated within the pump.
Other prior art designs utilize a stand alone component, e.g. an end cover, attached to the motor to house the relief valves. The attachment of the end cover, with relief valves, will dissipate these pressure surges. However the end cover, with incorporated relief valves, adds an unwanted length to the motor. The present invention overcomes the above obstacles by incorporating the relief valves directly in the housing of the motor.